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"How much did Father Christmas's sled cost? Zero, it was on the house."

This quip is greeted with moans that resonate through a storage facility in London.

We're at a joke-testing session with a firm that makes products for social events. Its catalogue includes Christmas crackers.

The firm's founder smiles, almost sheepishly at the joke. But the joke has made the cut and will feature in future crackers.

"You measure the gag by the volume of moans and the intensity of the groans around the table," the founder explains.

The secret to a great Christmas cracker joke is not the same as a stand-up gag per se. It is all about the setting - in this instance, the communal laughter of the Christmas meal with grandparents, kids and possibly friends.

"The goal is for the gag to be a thing that brings the child together with the 80-year-old," she states.

The Neuroscience Of Shared Amusement

Gathering to experience shared amusement is not only ancient, scientists say, it is likely to be older than humanity.

"Therefore when you are chuckling with others around the holiday table you are dropping into what's very likely a truly primordial mammalian social vocalisation," says a neuroscience expert.

Communal amusement, she says, aids in forge and strengthen social bonds between people.

Researchers have discovered that a absence of such social exchanges can significantly harm mental and physical well-being.

"The people you converse with, and laugh with, it results in increased levels of 'happy chemical' uptake," she continues.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce stress and pain and in reaction to pleasurable activities, such as laughing with friends over a particularly awful festive cracker gag.

"You're not just laughing at a silly pun with a Christmas cracker," the expert states. "You are in fact performing a lot of the really vital task of building, preserving the connections you have with the people you care about."

What Happens In the Brain?

But what is actually happening within the brain when we hear a gag?

An awful lot happens in reaction to humour, it transpires.

Using brain scanning technology, a kind of brain scanner which shows which parts of the mind are more active, researchers have been able to map the areas that receive more blood flow.

The research involves imaging the brains of healthy participants and then exposing them to a database of funny words, paired with either a non-emotional sound, or pre-recorded chuckles.

"During the study we observed a really interesting activation pattern of neural activity," says the neuroscientist.

A joke activates not just the parts of the mind responsible for auditory processing and understanding speech, but also neural regions associated with both preparation and starting movement and those linked to vision and memory.

Combine all of this as a whole, and people listening to a joke have a complex series of brain responses that underpin the laughter we hear.

The Infectious Power of Chuckles

Researchers found that when a funny phrase is paired with laughter there is a greater reaction in the brain than the same word when followed by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to move your face into a smile or a laugh," the professor explains.

It indicates we are not just responding to humorous jokes, they are reacting to the amusement that accompanies them.

Amusement, says the expert, can be contagious.

So what does this imply for the chuckles found at a Christmas gathering?

"People laugh harder when you are familiar with others," she says, "and you laugh further when you are fond of them or care for them."

When it comes to Christmas cracker puns, she says, the positive factor is more likely to be triggered not by the gag itself, but from the reaction to it.

"The laughter is key. The joke is the terrible holiday cracker joke, and it's just a pretext to laugh together."

The Quest for the Ideal Cracker Joke

Will we ever discover the perfect gag?

Likely not, but that has not stopped researchers from attempting to.

In 2001, a professor established a research search for the planet's most humorous joke.

More than 40,000 jokes submitted, with scores lodged by 350,000 people around the world, he has a better idea than most as to what works and what fails.

The ideal Christmas cracker pun must be short, he says.

"But they also need to be bad gags, puns that cause us to moan," he adds.

The increasingly "terrible" the joke, he states the more effective.

"This is because if nobody finds it funny – it's the joke's fault, not yours.

"What's interesting about the holiday cracker puns is that not one person considers them funny.

"It creates a common moment around the gathering and I think it's lovely."